Does not really matter on the volume of the hydro flask. The mouth size is what matters….I have seen Standard and Wide mouth/capsize for hydroflask. This cap will work with standard size cap/mouth size. It works awesome!

If the neck of the flask is small then this does work. I had bought one for my bf because I got him a 24 oz flask with a small neck but he hated the screw cap because it made it hard to drink from, so I looked all around locally but couldn’t find anything for a small neck straw cap until I found it on Amazon & just like that I fitted and now he is happy with it!!!